Multiple pass optical absorption cell

ABSTRACT

A multiple pass optical absorption cell of the type having a pair of objective mirrors and a field mirror, all having substantially the same radius of curvature and with the entrance and exit windows positioned to one side of the field mirror. In this invention, the two objective mirrors are positioned on a common mount with a fixed predetermined distance separating their centers of curvature. Path length is changed by rotation of the common mount, whereby the distance between the centers of curvature remains fixed. The foregoing abstract is not to be taken either as a complete exposition or as a limitation of the present invention, and in order to understand the full nature and extent of the technical disclosure of this application, reference must be had to the following detailed description and the accompanying drawings as well as to the claims.

United States atent n91 Gilby [54] MULTIPLE PASS OPTICAL ABSORPTION CELL[451 Apr. 10, 1973 Primary ExaminerRonald L. Wibert AssistantExaminerConrad Clark Inventor: Anthony C. Gilby, Darien; James Att0meyGarld Bramblett et Alan Horton Trumbull, both f Com 57 ABSTRACT Amultiple pass optical absorption cell of the type hav- [l3] Asslgnee' gf g Corpomnon South ing a pair of objective mirrors and a field mirror,all orw having substantially the same radius of curvature and [22]Filed: Feb. 14, 1972 with the entrance and exit windows positioned toone side of the field mirror. In this invention, the two ob- [211 App!225858 jective mirrors are positioned on a common mount with a fixedpredetermined distance separating their [52] U.S. Cl ..356/244, 356/51centers f curvature. P h ng h i hang y r a- [51] Int. Cl. ..G0ln 21/16,GOln 21/36 ticn cf the common mount, whereby the ta [58] Field of Search..356/244, 246, 51; between the centers f c r ure r mains fixed.

' 350/293; 330/43 The foregoing abstract is not to be taken either as acomplete exposition or as a limitation of the present [56] ReferencesC'ted invention, and in order to understand the full nature UNITEDSTATES PATENTS and extent of the technical disclosure of thisapplication, reference must be had to the following detailed descriptionand the accompanying drawings as well as to the claims.

10 Claims, 7 Drawing Figures ja w .m 1 (Ii:

PATENTED APR 1 01975 SHEET 2 OF 2 MULTIPLE PASS OPTICAL ABSORPTION CELLBACKGROUND OF THE INVENTION It is often desired to measure theabsorption spectra of small concentrations of gas or vapor phasecomponents of a sample mixture. Many such applications exist, one of themost common, for example, being found in the field of air pollutionmeasurement and.

control. For making such measurements, it is common to utilize aspectrometer having as an accessory, or as an integral part thereof, along path length gas cell. A cell of this type normally comprises anelongated housing having radiation entrance and exit windows and mirrorspositioned within to direct the radiation through multiple traversals togive a long effective path length sufficient to obtain the desiredabsorption.

The traditional multiple reflection cell design was described by John U.White in an article entitled Long Optical Paths of Large Apertureappearing at page 285, Vol. 32, of the Journal of the Optical Society ofAmerica in May, 1942. The basic principle involved the use of threemirrors. A pair of spherical objective mirrors are positioned at one endof the cell and a single spherical field mirror at the other. Allmirrors have the same radius of curvature and the separation of thefield mirror from the objective mirrors is approximately equal to theradius of curvature. The entrance mirror is positioned at one side ofthe field mirror and the exit window is positioned at the other side ofthe same mirror. The objective mirrors are rotatable relative to oneanother and the overall path length is determined by the spacing betweentheir centers of curvature. The intermediate images formed on the fieldmirror are positioned along a single horizontal line. The field mirrorassures that all the energy of the beam which leaves the first objectivemirror returns to the second objective mirror, and vice versa, even forlarge entrance image sizes. The White design permitted a beam of largeoptical aperture to be folded many times in a relatively short cell withthe main energy loss being due to the mirror surface reflectivity.

An article by T. H. Edwards entitled Multiple- Traverse Absorption CellDesign appears at page 98 of Vol. 51, No. 1, Journal of the OpticalSociety of America, Jan. 1961. The article describes a variation of theoriginal White design wherein the entrance and exit windows arepositioned above a line passing through the centers of curvature of theobjective mirrors. By means of this arrangement, two rows ofintermediate images are formed on the field mirror. As pointed out byboth White and Edwards, astigmatism is the principle aberration to beexpected, due to the substantial off-axis positions of the entrance andexit images. Edwards describes the design parameters contributing toastigmatism and suggests cutting down the field mirror to reduce thedistance between the entrance and exit slits.

An article by H. M. Pickett, G. M. Bradley, and H. L. Strauss White ANew with Type Multiple Pass Absorption Cell appears at page 2,397, Vol.9, No. of Applied Optics, Oct. 1970. This article describes stillanother variation of the original White system wherein the entrance andexit images are positioned one above the other at one edge of the fieldmirror. This has the advantage of reducing the minimum path lengthobtainable to twice the physical length of the cell. In the previouslydescribed arrangements, the minimum path length was four times thelength of the cell.

In all of the foregoing arrangements, the objective mirrors arerotatable relative to one another and the distance between their centersof curvature on the field mirror determines the number of traversals.Such an arrangement results in a number of disadvantages. For example,such cells have been notorious for difficulties in stability andalignment. This is due to the fact that slight misalignment of theobjective mirrors is cumulative and increases with the number ofreflections.

As pointed out above, the most serious aberration of the prior art cellsis astigmatism. This is due to the sub stantial off-axis positions ofthe entrance and exit images with the result that, even for small pathlengths, substantially the entire width of the field mirror is employed.

Furthermore, in the prior art designs there is no linear relationshipbetween the rotation of the adjustable objective mirror and the overallpath length. This results in difficulty in adjusting the cell for thedesired length.

Accordingly it is a primary object of the present invention to provide amultiple path optical absorption cell wherein possible misalignment ofthe objective mirrors is substantially eliminated.

Another objective is to minimize astigmatism by utilizing only as muchof the field mirror width as is required to obtain the path lengthdesired.

Another objective is to provide a cell having a linear relationshipbetween objective mirror rotation and cell path length to therebysimplify adjustment of the cell to the desired path length.

Still other objects, features, and advantages will be apparent from thefollowing description and appended claims.

SUMMARY OF THE INVENTION The objects of this invention are achieved in amultiple path optical absorption cell of the type including an elongatedhousing containing two adjacent spherical objective mirrors at one endand a spherical field mirror at the other end with the radiationentrance and exit windows positioned at the same side of the fieldmirror. The cell is improved by provision of a common mount supportingboth of the objective mirrors with their centers of curvature separateda preselected distance. Means are provided for rotating the common mountto move the centers of curvature relative to the field mirror andthereby vary the optical path length through the cell.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional plan viewof a gas cell constructed in accordance with the present invention;

FIG. 2 is an enlarged illustration of a portion of the objective mirrorsand the objective mirror mount taken along the line 2-2 of FIG. 1;

FIG. 3 is an end view of the objective mirrors of FIG. 2 taken along theline 33 of FIG. 2;

FIGS. 4-6 are illustrations of the images formed on the field mirror fordifferent rotational positions of the objective mirrors; and

FIG. 7 is a schematic elevational view of the cell of this invention incombination with a spectrometer.

DESCRIPTION OF THE PREFERRED EMBODIMENT With particular reference toFIG. 1, there is illustrated a gas cell in accordance with thisinvention comprising an elongated housing 10. Internally of thishousing, at one end, is mounted a concave spherical field mirror 12.Vertically aligned openings adjacent one edge of the field mirrorcontain lenses defining an entrance window 14 (FIG. 7) and an exitwindow 16.

The opposite end of the cell housing defines an opening 18 which isclosed by a flanged plate 20 secured by screws 22. Mounted on oppositeedges of plate 20 are a pair of L-shaped mounting brackets 24, 26.(FIGS. 2 and 3). The brackets 24, 26 are similar and include spaced,parallel arms 23, each defining a partial circular recess 30 on itsinner surface. Mounted within each recess is a disc 32 secured by ascrew 34. A portion of each disc protrudes from its recess and defines aradial adjustinghole 36. Defined on the inner surface of each disc is asmall depression 38 for retaining a ball bearing 40.

Mounted between the brackets 24, 26 is a mirror mount 42 havingupstanding lugs 44, 46. The outer surface of each lug defines adepression 48, each receiving a respective ball bearing 40. Across theface of the mirror mount 42 is a milled slot 50 which slightly weakensthe mount 42 to permit a limited hinging action between the two portionsof the mount. Secured to one end of the mirror mount, on its backsurface, is one end of an angled adjusting bar 52. The other end of theadjusting bar is spaced from the back surface of the mirror mount 42, asshown in FIG. 1, and carries an adjusting screw 54. One end of a leafspring 56 is secured to the adjusting bar, with its other end bearingagainst the plate 20.

It will now be apparent that the mirror mount 42 is pivoted between thebearings 40. As viewed in FIG. 1, the spring 56 tends to rotate themirror mount in a counterclockwise direction against a path lengthadjustment screw 58. An opening 60 closed by a suitable plug 62 providesaccess to adjusting screw 54. Secured to the front surface of mirrormount 42 are spherical objective mirrors 64, 66.

FIG. 7 illustrates schematically the gas cell of the invention combinedwith an infrared spectrometer including a source 68, a filter wheel 70,a chopper '72, an imaging lens 74, a detector 76, and an electronicboard As stated, the field mirror 12 and the objective mirrors 64, 66are spherical mirrors and all have essentially the same radius ofcurvature. Furthermore, the field mirror and the objective mirrors areseparated by such radius. Referring to FIG. 1, the center of curvatureof field mirror 12 is shown as 120 and is located at the juncture of theobjective mirrors. The center of curvature 64c of objective mirror 64and the center of curvature 660 of objective mirror 66 are located onthe surface of field mirror 12, a preselected distance apart. It willalso be apparent that adjustment of path length adjusting screw 58 willpivot the objective mirrors on their common mount, causing the positionsof the centers of curvature 64c, 660 to be selectively positioned alongthe surface of the field mirror 12. However, the distance between thecenters of curvature remains fixed, in contrast to prior art devices.

OPERATION The operation of the invention will be best understood byreference to FIGS. 1 and 46. FIGS. 4-6 illustrate the entrance and exitimages, each of which is adjacent to field mirror 12, and theintermediate images which are formed on the field mirror with changes ofrotation of the objective mirrors. In these illustrations, the centersof curvature 64c, 660 of the objective mirrors are separated by one-halfof the beam image width. Referring first to FIG. 4, the entrance image 1is illustrated as it would normally be placed adjacent the edge of thefield mirror 12. The center of curvature 64c of objective mirror 64 isabove, but aligned with, the entrance image. Radiation from the entranceimage fills the spherical mirror 64 and is returned to form exit image2. This position results in the shortest possible path length, which istwice the length of the cell. By advancing path length adjustment screw58, the centers of curvature may be advanced to the positionsillustrated in FIG. 5. In this position, the center of curvature 64c ison the edge of field mirror 12 while center 66c remains one-half theimage width to the right. Entrance image 1 is received by the mirror 64which forms a second image 2 on the field mirror. Image 2 is refocusedby mirror 66 to form image 3 which, in turn, is refocused by mirror 64to form exit image 4. In this arrangement the path length is six timesthe cell length.

It is important to note that all images are closely packed at the sameedge of the field mirror 12. Accordingly, the displacement of the imagesrelative to the axes of the objective mirrors is held to a minimum whichresults also in minimum astigmatism. This close packing continues witheach further adjustment, with only the minimum required width of thefield mirror 12 being utilized. FIG. 6, for example, illustrates a moreadvanced position of the path length adjustment screw to form a total of20 images on the field mirror 12, the exit image being number 22. Atthis position the path length is 42 times the length of the cell and itwill be noted that the close packing of the images has continued.

The rotational axis of the objective mirrors formed by ball bearings 40is not centered on the objective mir rors. If it is so centered, image 3falls off into the entrance aperture when there are a large number ofpasses, causing a consequent loss of energy. The reason for this isrelated to the build-up of aberrations in the cell as more images arepacked onto the field mirror. By locating the rotation axis off center,a compensating shift in image positions is induced so that the images donot track across the field mirror to any significant extent as pathlength is changed. The off-center displacement has been empiricallydetermined. In one embodiment wherein the distance between the objectivemirrors and field mirror is 375 mm and the width of each objectivemirror is mm, the displacement is 12 mm.

By mounting the ball bearing pivots in the discs 32, fine focusing ofthe images may be obtained. This is accomplished by loosening the screws34 and inserting a wire or other tool into the holes 36. Rotation of thediscs 32 then results in a forward or backward translation of the mirrormount 42 and objective mirrors.

It is also possible to make small adjustments to the angle between theobjective mirrors. This is accomplished by inserting a tool through theopening 60 and rotating the adjusting screw 54, causing a change in theangle formed by mount 42 at slot 50. This would not be done duringnormal operation.

To summarize, the following advantages are obtained with the describedinvention:

1. Both objective mirrors are held rigidly together. This preventsmisalignment of one objective mirror relative to the other, causingerrors which accumulate with increased path length. Slight misalignmentof the pair of mirrors together, however, is not cumulative.

2. Because both objective mirrors are held together as a unit, there isa linear relationship between the adjusting screw position and cell pathlength. This is an important practical advantage, as the path length canbe read out on a simple counter on the adjusting screw.

3. There is a well defined exit aperture width for all path lengths and,thus, a successful transfer of the beam to the detector. Slight sidewaysspreading of the image on the field mirror caused by astigmatism doesnot result in energy loss because the exit aperture remains constant. v

4. Because only the width of field mirror required to pack a givennumber of images is used, the aberrations in the final output image arekept as small as possible.

It will now be apparent that all the objectives of this invention havebeen obtained by means of the described construction. It will also beapparent that a number of variations and modifications may be made inthis invention without departing from its spirit and scope. Accordingly,the foregoing description is to be construed as illustrative only,rather than limiting. This invention is limited only by the scope of thefollowing claims.

We claim:

1. In a multiple pass optical absorption cell of the type including anelongated housing containing first and second adjacent sphericalobjective mirrors at one end thereof, a spherical field mirror at theother end thereof, and radiation entrance and exit windows at the sameside of said field mirror, the improvement which comprises: a commonmount supporting both of said objective mirrors with their centers of Icurvature separated a preselected distance; and means for rotating saidmount to move said centers of curvature relative to said field mirror tovary the optical path length through said cell.

2. The improvement of claim 1 wherein said rotating means comprises apair of aligned pivots displaced from the juncture of said objectivemirrors.

3. The improvement of claim 2 wherein each of said pivots iseccentrically mounted in a selectively rotatable disc.

4. The improvement of claim 1 wherein said mount comprises a platemember having a hinge portion substantially at the juncture of saidobjective mirrors.

5. The improvement of claim 4 wherein said hinge portion is a thinnersection of said plate member.

6. The improvement of claim 4 wherein said mount comprises means foradjustably varying the angle formed by said plate member about its hingeportion to adjust said preselected distance.

. The improvement of claim 6 wherein said varying means comprises: anelongated member having one end fixedly secured to one hinged portion ofsaid plate member and having its other end spaced from the other hingedportion; and adjusting screw means threaded through said other end inengagement with the other hinged portion of said plate member.

8. The improvement of claim 1 wherein said rotating means comprises:means for resiliently urging said mount in a first direction ofrotation; and an adjustable stop member limiting said rotation.

9. the improvement of claim 8 wherein said resilient means is a spring.

10. The improvement of claim 8 wherein said stop member is a screwsupported in said housing.

t i I

1. In a multiple pass optical absorption cell of the type including anelongated housing containing first and second adjacent sphericalobjective mirrors at one end thereof, a spherical field mirror at theother end thereof, and radiation entrance and exit windows at the sameside of said field mirror, the improvement which comprises: a commonmount supporting both of said objective mirrors with their centers ofcurvature separated a preselected distance; and means for rotating saidmount to move said centers of curvature relative to said field mirror tovary the optical path length through said cell.
 2. The improvement ofclaim 1 wherein said rotating means comprises a pair of aligned pivotsdisplaced from the juncture of said objective mirrors.
 3. Theimprovement of claim 2 wherein each of said pivots is eccentricallymounted in a selectively rotatable disc.
 4. The improvement of claim 1wherein said mount comprises a plate member having a hinge portionsubstantially at the juncture of said objective mirrors.
 5. Theimprovement of claim 4 wherein said hinge portion is a thinner sectionof said plate member.
 6. The improvement of claim 4 wherein said mountcomprises means for adjustably varying the angle formed by said platemember about its hinge portion to adjust said preselected distance. 7.The improvement of claim 6 wherein said varying means comprises: anelongated member having one end fixedly secured to one hinged portion ofsaid plate member and having its other end spaced from the other hingedportion; and adjusting screw means threaded through said other end inengagement with the other hinged portion of said plate member.
 8. Theimprovement of claim 1 wherein said rotating means comprises: means forresiliently urging said mount in a first direction of rotation; and anadjustable stop member limiting said rotation.
 9. the improvement ofclaim 8 wherein said resilient means is a spring.
 10. The improvement ofclaim 8 wherein said stop member is a screw supported in said housing.